| Summary: | <p>Gamma decays of excited states of <sup>11</sup>B, <sup>14</sup>C, <sup>18</sup>Ne, <sup>19</sup>Ne, and <sup>26</sup>Si have been studied experimentally and the results used to test the predictions of various nuclear models. The phase-consistent Rose and Brink treatment of electromagnetic transitions was used throughout.</p> <p>States of <sup>11</sup>B and <sup>14</sup>C were studied using the reactions <sup>12</sup>C(t,α)<sup>11</sup>B and <sup>12</sup>C(t,p)<sup>14</sup>C in conjunction with a conventional charged particle-gamma ray coincidence angular correlation technique. For the <sup>11</sup>B, 4.44 MeV, J=<sup>5</sup>⁄<sub>2</sub> level previous measurements of the ground state decay mixing ratio were confirmed (δ = −0.19 ± 0.015). For the <sup>11</sup>B, 5.02 MeV level the spin was found to be <sup>3</sup>⁄<sub>2</sub>; the mixing ratio of the (88 ± 2.5%) transition to the ground state was measured to be δ = 0.03 ± 0.05, and that of the 5.02 → 2.12 MeV transition δ = −0.05 ± 0.2. The results for both <sup>11</sup>B levels are in substantial agreement with the predictions of intermediate coupling calculations.</p> <p>The results for <sup>14</sup>C levels are summarised in the table (where parities and final state spins nave been taken from the literature).</p> <table> <caption><sup>14</sup>C Results</caption> <tr> <th>Transition (MeV)</th> <th>Initial J<sup>π</sup></th> <th>Final J<sup>π</sup></th> <th>Branch (%)</th> <th>Mixing Ratio δ</th> </tr> <tr> <td>6.72 → g.s.</td> <td>3<sup>−</sup></td> <td>0<sup>+</sup></td> <td>97.3 ± 1.0</td> <td>0</td> </tr> <tr> <td>6.72 → 6.09</td> <td>3<sup>−</sup></td> <td>1<sup>−</sup></td> <td>2.7 ± 1.0</td> <td>−</td> </tr> <tr> <td>7.01 → g.s.</td> <td>2<sup>+</sup></td> <td>0<sup>+</sup></td> <td>98.6 ± 0.7</td> <td>0</td> </tr> <tr> <td>7.01 → 6.09</td> <td>2<sup>+</sup></td> <td>1<sup>−</sup></td> <td>1.4 ± 0.7</td> <td>−</td> </tr> <tr> <td>7.35 → g.s.</td> <td>2<sup>−</sup></td> <td>0<sup>+</sup></td> <td>14 ± 4</td> <td>0</td> </tr> <tr> <td>7.35 → 6.09</td> <td>2<sup>−</sup></td> <td>1<sup>−</sup></td> <td>52 ± 5</td> <td>−0.04 ± 0.09</td> </tr> <tr> <td>7.35 → 6.72</td> <td>2<sup>−</sup></td> <td>3<sup>−</sup></td> <td>34 ± 4</td> <td>0.07 ± 0.3</td> </tr> </table> <p>The results are compared to predictions of True's model of <sup>14</sup>C, in which the active particles are two neutrons in the 1p<sub><sup>1</sup>⁄<sub>2</sub></sub>, 2s<sub><sup>1</sup>⁄<sub>2</sub></sub>, 1d<sub><sup>3</sup>⁄<sub>2</sub></sub> and 1d<sub><sup>5</sup>⁄<sub>2</sub></sub> shells. For the 6.72 MeV level agreement is good, but the 7.01 → 6.09 MeV and the 7.35 MeV → g.s. decays are predicted too strong. The explanation probably lies in excitation of the 1p<sub><sup>3</sup>⁄<sub>2</sub></sub> core particles, as considered by Sebe.</p> <p>The bound states of <sup>18</sup>Ne were studied using the <sup>16</sup>O(<sup>3</sup>He,n)<sup>18</sup>Ne reaction in conjunction with a neutron-gamma ray coincidence angular correlation technique. Neutrons were detected in a large liquid scintillator using pulse shape discrimination. Gamma ray detection in large-volume Ge(Li) counters enabled separation of the level-decays under study. The missing J<sup>π</sup> = O<sup>+</sup> level was found at 3.58 MeV and its origin in <sup>18</sup>Ne confirmed. The decay schemes and mixing ratios are shown below. The level spins and parities above 2 MeV have been deduced by synthesis of the present results with published <sup>18</sup>Ne level lifetimes and double stripping studies, and by analogies with the mirror nucleus <sup>18</sup>O.</p> <table> <caption><sup>18</sup>Ne Results</caption> <tr> <th colspan="3">Transition (MeV)</th> <th>Initial J<sup>π</sup></th> <th>Final J<sup>π</sup></th> <th>Branch (%)</th> <th colspan="3">Mixing Ratio δ</th> </tr> <tr> <td>3.38</td><td>→</td><td>0</td> <td>4<sup>+</sup></td> <td>0<sup>+</sup></td> <td>< 4</td> <td colspan="3"> </td> </tr> <tr> <td> </td><td>→</td><td>1.89</td> <td>4<sup>+</sup></td> <td>2<sup>+</sup></td> <td>100</td> <td>+0.06</td><td>±</td><td>0.07</td> </tr> <tr> <td>3.58</td><td>→</td><td>0</td> <td>0<sup>+</sup></td> <td>0<sup>+</sup></td> <td>< 17</td> <td colspan="3"> </td> </tr> <tr> <td> </td><td>→</td><td>1.89</td> <td>0<sup>+</sup></td> <td>2<sup>+</sup></td> <td>100</td> <td colspan="3"> </td> </tr> <tr> <td>3.62</td><td>→</td><td>0</td> <td>2<sup>+</sup></td> <td>0<sup>+</sup></td> <td>< 9</td> <td colspan="3"> </td> </tr> <tr> <td rowspan="2"> </td><td rowspan="2">→</td><td rowspan="2">1.89</td> <td rowspan="2">2<sup>+</sup></td> <td rowspan="2">2<sup>+</sup></td> <td rowspan="2">100</td> <td rowspan="2">−1.1</td><td>+</td><td>1.0</td> </tr> <tr> <td>−</td><td>0.3</td> </tr> </table> <p>The <sup>18</sup>Ne results are shown to be in good accord with the predictions of the Benson-Irvine model which includes a deformed 4-particle-2-hole component in the wavefunctions along with the conventional two-proton (sd)<sup>2</sup> configurations. <p>Decays of excited states of <sup>19</sup>Ne below 3 MeV were similarly studied using the <sup>19</sup>(F(p,n)<sup>19</sup>Ne reaction. By synthesis with published lifetimes, and by analogy with levels of the mirror nucleus <sup>19</sup>F, the results in the table were obtained:</p> <table> <caption><sup>19</sup>Ne Results</caption> <tr> <th>Transition (MeV)</th> <th>Initial J<sup>π</sup></th> <th>Final J<sup>π</sup></th> <th>Branch (%)</th> <th>Mixing Ratio (δ)</th> </tr> <tr> <td>1.51 → g.s.</td> <td><sup>5</sup>⁄<sub>2</sub><sup>−</sup></td> <td><sup>1</sup>⁄<sub>2</sub><sup>+</sup></td> <td>< 3</td> <td>−</td> </tr> <tr> <td>1.51 → 0.24</td> <td><sup>5</sup>⁄<sub>2</sub><sup>−</sup></td> <td><sup>5</sup>⁄<sub>2</sub><sup>+</sup></td> <td>12 ± 7</td> <td>−</td> </tr> <tr> <td>1.51 → 0.28</td> <td><sup>5</sup>⁄<sub>2</sub><sup>−</sup></td> <td><sup>1</sup>⁄<sub>2</sub><sup>−</sup></td> <td>88 ± 3</td> <td>0.1 ± 0.2</td> </tr> <tr> <td>1.54 → g.s.</td> <td><sup>3</sup>⁄<sub>2</sub><sup>+</sup></td> <td><sup>1</sup>⁄<sub>2</sub><sup>+</sup></td> <td>< 7</td> <td>−</td> </tr> <tr> <td>1.54 → 0.24</td> <td><sup>3</sup>⁄<sub>2</sub><sup>+</sup></td> <td><sup>5</sup>⁄<sub>2</sub><sup>+</sup></td> <td>100</td> <td>−</td> </tr> <tr> <td>1.54 → 0.28</td> <td><sup>3</sup>⁄<sub>2</sub><sup>+</sup></td> <td><sup>1</sup>⁄<sub>2</sub><sup>−</sup></td> <td>< 7</td> <td>−</td> </tr> <tr> <td>1.62 → g.s.</td> <td><sup>3</sup>⁄<sub>2</sub><sup>−</sup></td> <td><sup>1</sup>⁄<sub>2</sub><sup>+</sup></td> <td>20 ± 2</td> <td>−</td> </tr> <tr> <td>1.62 → 0.24</td> <td><sup>3</sup>⁄<sub>2</sub><sup>−</sup></td> <td><sup>5</sup>⁄<sub>2</sub><sup>+</sup></td> <td>10 ± 2</td> <td>−</td> </tr> <tr> <td>1.62 → 0.28</td> <td><sup>3</sup>⁄<sub>2</sub><sup>−</sup></td> <td><sup>1</sup>⁄<sub>2</sub><sup>−</sup></td> <td>70 ± 2</td> <td>−</td> </tr> <tr> <td>2.79 → g.s.</td> <td><sup>9</sup>⁄<sub>2</sub><sup>+</sup></td> <td><sup>1</sup>⁄<sub>2</sub><sup>+</sup></td> <td>< 10</td> <td>−</td> </tr> <tr> <td>2.79 → 0.24</td> <td><sup>9</sup>⁄<sub>2</sub><sup>+</sup></td> <td><sup>5</sup>⁄<sub>2</sub><sup>+</sup></td> <td>100</td> <td>0.0 ± 0.1</td> </tr> <tr> <td>2.79 → 0.28</td> <td><sup>9</sup>⁄<sub>2</sub><sup>+</sup></td> <td><sup>1</sup>⁄<sub>2</sub><sup>−</sup></td> <td>< 10</td> <td>−</td> </tr> <tr> <td>2.79 → 1.51</td> <td><sup>9</sup>⁄<sub>2</sub><sup>+</sup></td> <td><sup>5</sup>⁄<sub>2</sub><sup>−</sup></td> <td>< 12</td> <td>−</td> </tr> <tr> <td>2.79 → 1.54</td> <td><sup>9</sup>⁄<sub>2</sub><sup>+</sup></td> <td><sup>3</sup>⁄<sub>2</sub><sup>+</sup></td> <td>< 10</td> <td>−</td> </tr> <tr> <td>2.79 → 1.62</td> <td><sup>9</sup>⁄<sub>2</sub><sup>+</sup></td> <td><sup>3</sup>⁄<sub>2</sub><sup>−</sup></td> <td>< 10</td> <td>−</td> </tr> </table> <p>Levels of <sup>26</sup>Si were studied using the reaction <sup>24</sup>Mg(<sup>3</sup>He,n)<sup>26</sup>Si. Decay schemes and level energies were determined, and the Doppler-shift attenuation method was used to measure level lifetimes. The results are shown in the table.</p> <table> <caption><sup>26</sup>Si Results</caption> <tr> <th>Level Energy (keV)</th> <th colspan="3">Lifetime (ps)</th> <th>Transition to (keV)</th> <th>Branch (%)</th> <th>Initial J<sup>π</sup></th> </tr> <tr> <td rowspan="2">1795.9 ± 0.2</td> <td rowspan="2">1.38</td><td>+</td><td>0.7</td> <td rowspan="2">g.s.</td> <td rowspan="2">100</td> <td rowspan="2">2<sup>+</sup></td> </tr> <tr> <td>−</td><td>0.5</td> </tr> <tr> <td rowspan="2">2783.5 ± 0.4</td> <td rowspan="2">0.20</td><td>+</td><td>0.20</td> <td rowspan="2">g.s.</td> <td rowspan="2">30 ± 5</td> <td rowspan="2">2<sup>+</sup></td> </tr> <tr> <td>−</td><td>0.12</td> </tr> <tr> <td colspan="4"> </td> <td>1796</td> <td>70 ± 5</td> <td> </td> </tr> <tr> <td rowspan="2">3332.5 ± 0.3</td> <td rowspan="2">2.7</td><td>+</td><td>2.3</td> <td rowspan="2">1796</td> <td rowspan="2">100</td> <td rowspan="2">0<sup>+</sup></td> </tr> <tr> <td>−</td><td>1.0</td> </tr> <tr> <td>3756 ± 2</td> <td colspan="3">< 0.7</td> <td>1796</td> <td>70 ± 10</td> <td>(3<sup>+</sup>)</td> </tr> <tr> <td colspan="4"> </td> <td>2784</td> <td>30 ± 10</td> <td> </td> </tr> <tr> <td>3842 ± 2</td> <td colspan="3">−</td> <td>1796</td> <td>100</td> <td>(4<sup>+</sup>)</td> </tr> <tr> <td>4093 ± 3</td> <td colspan="3">−</td> <td>g.s.</td> <td>100</td> <td> </td> </tr> <tr> <td rowspan="2">4138 ± 1</td> <td rowspan="2">0.11</td><td>+</td><td>0.11</td> <td rowspan="2">g.s.</td> <td rowspan="2">20 ± 10</td> <td rowspan="2"> </td> </tr> <tr> <td>−</td><td>0.10</td> </tr> <tr> <td colspan="4"> </td> <td>1796</td> <td>80 ± 10</td> <td> </td> </tr> <tr> <td>4445 ± 3</td> <td colspan="3">< 0.5</td> <td>1796</td> <td>100</td> <td> </td> </tr> <tr> <td>4805 ± 2</td> <td colspan="3">< 0.1</td> <td>1796</td> <td>30 ± 10</td> <td> </td> </tr> <tr> <td colspan="4"> </td> <td>2784</td> <td>70 ± 10</td> <td> </td> </tr> </table> <p>J<sup>π</sup> for the 3756 and 3842 keV levels have been deduced from the observed decay widths and from mirror level arguments; others are from the literature.</p> <p>The results are compared to shell-model calculations for ten particles in the 2s<sub><sup>1</sup>⁄<sub>2</sub></sub>-1d<sub><sup>5</sup>⁄<sub>2</sub></sub> orbitals using the two~nucleon matrix elements of Wildenthal (from an effective interaction fit to level energies) and of Kuo (based on the realistic Hamada-Johnston potential). Neither calculation gives a good description), though the Kuo predictions are better overall. Discrepancies underline the importance of including the 1d<sub><sup>3</sup>⁄<sub>2</sub></sub> orbital in such calculations.</p></p>
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